This invention relates to the field of high intensity internal mixing machines of the batch type having a mixing chamber shaped to accommodate two counter-rotating, non-intermeshing, tangential winged rotors. The batch of ingredients to be mixed into a homogeneous mass is fed down into the mixing chamber through a vertical chute and is pushed down under pressure by a ram located in the chute. This ram is hydraulically, pneumatically or electrically driven. The lower face of the ram, when advanced down to its operating position during mixing of the batch, forms an upper portion of the mixing chamber. The homogeneous mixture produced is removed from the mixing chamber through a discharge opening at the bottom of the chamber, and a door associated with this opening is then closed in readiness for the next batch of ingredients to be introduced down through the chute. The homogenous mixture can also be removed in an alternative design by raising the hopper, tilting the mixing chamber and reversing the rotor direction. This batch mixer design is commonly referred to as a “tilt mixer.” The homogenous mixture can also be removed by a mechanism where in the hinged side of the mixer is opened allowing the mixture to be removed from the mixing chamber. This design is commonly referred to as a “swing or hinged side discharge” batch mixer.
Some internal batch mixing machines are designed with non-intermeshing rotors, and others have intermeshing rotors. Intermeshing rotors must always be driven at the same rotational speed in synchronized relationship; non-intermeshing rotors may be driven at the same rotational speed or at different rotational speeds for achieving different mixing and kneading effects. The present invention relates to the non-intermeshing type. The wings of the rotors have a generally helical configuration, and they produce high intensity mixing and homogenization by the co-operative interaction of their various forceful dynamic effects, as described later. For further information about such internal batch mixers, having non-intermeshing rotors, reference may be made to U.S. Pat. Nos. 1,200,070 and 3,610,585, assigned to predecessors of the present assignee, and to U.S. Pat. Nos. 4,744,668 and 4,834,543, assigned to the present assignee, and the disclosures of these patents are incorporated herein by reference as background information.
The wings of rotors employed in internal batch mixers have a generally helical configuration and they therefore produce high intensity mixing and homogenization by the co-operative interaction of the various forceful dynamic effects. Traditionally, the rotors in non-intermeshing type internal batch mixers have been driven at different speeds (non-synchronous rotor mixing). In more recent years, manufacturers have designed non-intermeshing type rotors that are intended to operate at equal speed (synchronous rotor mixing), such as for example, the rotors of the type disclosed in assignee U.S. Pat. No. 6,494,607, incorporated herein by reference thereto. Such rotors have produced increased distributive and dispersive mixing of materials in the internal batch mixers. Additionally, in order to decrease mixing cycle times, users of non-intermeshing type mixers have sought to increase the rotor speeds.
While the rams utilized in intermeshing rotor mixers have utilized a non-uniform shape for the portion of the ram that protrudes different distances into the mixer, the rams heretofore utilized in these latter non-intermeshing rotor type internal batch mixers have had a “flat bottom” that does not protrude at all into the mixing chamber or a “V” type shaped bottom that has a uniformly shaped bottom from end-to-end, with the apex of the “V” along the whole bottom surface of the ram and running parallel to the rotational axis of the rotors in the internal batch mixer, such that the ram protrudes into the mixing chamber of the internal batch mixer an equal amount along the entire length of the ram and thus essentially along the entire length of the rotors. It has now been discovered that due to the shape of the helix on the rotors, with each revolution of the rotors during the mixing cycle, material to be mixed in the mixing chamber of the internal batch mixer is forced against each quadrant of the ram at different times producing varying forces on the ram and causing cyclical rotational forces on the ram. While, for traditional non-synchronous driven rotor internal batch mixers operated at relatively low rotor rotation speeds these varying cyclical rotational forces have not posed significant difficulties, the inventors have found this not to be the case for synchronous driven rotor internal batch mixers operating with increased rotor speeds. In this latter case, the inventors have discovered that the combination of synchronous rotor design and increased rotor speed has produced a more aggressive flow of material in the mixing chamber and significantly increased the cyclical rotational forces on the “V” bottom of the ram. As the rotors in the mixing chamber turn, the material being mixed is moved across resulting in substantial, significant noise and vibration of the ram against the sides of mixing chamber and housing associated therewith during operation of the internal batch mixer and resulting in significant wear on the ram and throat of the mixing chamber. With these new rotors and increased rotor speed the material being mixed is moved axially to the rotor end plates causing the ram to move from side to side. Due to the rotor wing interaction (rotor to rotor) the material being mixed is also transferred from one mixing chamber cavity to the other. If the mix is high in viscosity, the ram is moved up and down in the throat opening of the mixer as the material transfer is occurring. If the mix is low in viscosity, the “V” of the ram bottom restricts rotor-to-rotor exchange of the mix and the product mix turns with the rotor in the each cavity. All this movement creates noise, wear and mechanical impact damage. With the “flat bottom” ram design, material is not pushed into the mixing chamber between the rotors and has a tendency to stagnate under the flat section of the bottom of the ram, never entering the mixing chamber. There is, therefore, a need for an improved ram for use in counter-rotating, non-intermeshing rotor internal batch mixers, and particularly an improved ram for such mixers that provides at least equivalent or better distributive and dispersive mixing as obtained with the standard “V” ram, but that significantly reduces or substantially eliminates the cyclical rotational forces on the ram that causes the aforementioned noise, wear and damage caused by motion of the ram.